The present invention relates generally to magnetic resonance imaging (MRI), and more particularly relates to the cancellation of ghost artifacts in MRI imaging caused by a variety of distortion mechanisms.
Magnetic Resonance Imaging is based on the absorption and emission of energy in the radio frequency range. To obtain the necessary MR images, a patient (or other target) is placed in a magnetic resonance scanner. The scanner provides a uniform magnetic field that causes individual magnetic moments of spins in the patient or target to align with the magnetic field. The scanner also includes multiple coils that apply a transverse magnetic field. RF pulses (called xe2x80x9cshotsxe2x80x9d) are applied to the coils that cause the aligned moments to be rotated or tipped. In response to the RF pulses, a signal is emitted by the excited spins that is detected by receiver coils.
The resulting data obtained by the receiver coils corresponds to the spatial frequency domain and is called k-space data. The k-space data includes multiple lines called phase encodes or echoes. Each line is digitized by collecting a number of samples (e.g., 128-256). A set of k-space data is acquired for each image frame, and each k-space data set is converted to an image by passing the data through a fast Fourier transform (FFT). FIG. 1A shows an example of a full k-space data set with all of the phase encodes (1, 2, 3 . . . N) acquired.
In several applications of MRI, a time series or sequence of images are obtained in order to resolve temporal variations experienced by the imaged object. For example, in cardiac imaging it is desirable to obtain a sequence of images to study the dynamic aspects of the heart. Unfortunately, image distortion such as ghost artifacts or blurring may interfere with the ability to properly interpret the image. An artifact is a feature that appears in the resultant image even though it is not actually present in the target object. Amplitude and/or phase distortion in the acquired k-space data causes distortion in the resultant reconstructed image. The order of k-space acquisition (phase encode order) is an important factor in determining the type of image distortion. Periodic distortion of k-space data causes periodic ghosts artifacts. A ghost artifact appears as part of the target object shifted an offset amount and superimposed on the final image.
There are a wide variety of mechanisms that cause distortion of the acquired k-space data and that may result in ghost and/or blurring artifacts. If the phase encode order results in distortion that is periodic or has a periodic component, the image will have periodic ghost artifacts. In this context, distortion is described herein as periodic if it has a periodic component (which causes image domain ghosts), even if the distortion is not purely periodic since it may contain other non-periodic components. Examples of distortion mechanisms include off-resonance due to chemical shift or susceptibility variation, flow (e.g., blood flow), motion of the imaged object (e.g., breathing, heart, etc.), EPI delay or phase misalignment, and T2* amplitude decay. Ghosts may also result from periodic undersampling of k-space, which is used in a number of reduced field of view methods for accelerated imaging.
Distortion may be space invariant or space variant. Space invariant distortion refers to the case where each pixel in the image has been affected by the same distortion, while the more general case of space variant distortion refers to the case where the distortion may vary depending on the pixel location. With a space invariant ghost, all pixels in the image have a corresponding ghost with a fixed separation and same relative amplitude. In the case of space variant ghost distortion, the relative amplitude and/or separation of the ghost may depend on the pixel location.
FIG. 2A shows an example of multi-shot echo-planar imaging (EPI) with a non-interleaved phase encode order that cause distortion. The phase encodes are shown indicating the direction of a scan (indicated by arrow), such as shown at 10. As can be seen, the echoes are taken sequentially from each shot (e.g., echo 1, echo 2, echo 3, etc.). In this example, each shot has 4 echoes. Because the echo time (TE) for each echo is different, the amplitude and phase are different for each echo, which creates a distortion of the k-space data. Consequently, this non-interleaved ordering causes periodic distortion of the k-space data, which causes periodic ghosts in the resultant reconstructed image. For this reason, multi-shot non-interleaved phase encode ordering is not typically used to avoid ghost artifacts.
In this multi-shot EPI example, many prior art techniques eliminate the ghosts by acquiring the k-space data using an interleaved phase encode order to ensure that the distortion is not periodic and is a slowly varying function of k-space. Furthermore, a technique known as echo-shifting is also used to linearize the echo time (TE) versus phase encode number (ky) which also reduces blur distortion at the cost of increased overall acquisition time.
FIG. 2B shows an example of an interleaved phase encode order. In the illustrated example, each shot has four echoes. The line of k-space are acquired in an interleaved manner such that groups of adjacent lines in k-space are acquired at the same echo time (TE). For example, the first echoes from each shot are grouped together, as shown at 12. Likewise, all of the echos from the second shot are grouped together, as shown at 14. Grouping together similar echoes in this interleaved manner eliminates the rapid variation of echo time versus k-space, and, therefore, eliminates widely spaced ghost artifacts in favor of a more subtle blurring and/or geometric distortion.
Echo-planar imaging (EPI) is used in many MR rapid imaging applications and ghost reduction for EPI has received considerable attention. Many techniques on the prior art are based on compensating (equalizing) periodic k-space distortion. These methods first estimate the periodic phase (or other) distortion, and then apply compensating phase function to eliminate or reduce the distortions. Numerous schemes have been developed for estimating the phase errors. However, these methods only cope with the case of space invariant distortion, therefore, residual distortion will remain, due to a number of space variant mechanisms that cannot be compensated for in this manner. Ghost artifacts due to local effects such as flow and off-resonance are space variant and are not mitigated by k-space phase compensation methods.
Methods have been developed which address certain cases of space variant distortion, such as local off-resonance effects. These rely on estimating the space variant distortion by means of a measurement of the field map, followed by applying the inverse to remove the space variant distortion. It is difficult to obtain accurate field maps, particularly in cases where the susceptibility (field) is time varying, such as in cardiac imaging applications. These methods are often quite sensitive to error cause by noise.
Phase array combining methods have been used for accelerated imaging (e.g., methods known as SENSE and SMASH) to cancel space invariant ghosts that arise from periodic undersampling. SMASH has also been applied to more general EPI ghost cancellation but still only handles space invariant distortion. An example of a technique using the SENSE method applied to single shot EPI ghost cancellation is shown in Kuhara et al., A Novel EPI Reconstruction Technique using Multiple RF Coil Sensitivity Maps. This application acquired multi-coil full field-of-view (FOV) k-space data and separates the k-space data into even and odd lines. The even lines are passed through a first fast Fourier transform image reconstruction component, while the odd lines are passed through a second fast Fourier transform component. The even and odd lines are separately processed using the SENSE method. The outputs of each of the separate SENSE reconstructions are then non-coherently combined to obtain the final image with ghost artifacts cancelled. In this latter method, magnitude combining was used which precluded this method to be used in conjunction with techniques which required preserving phase, such as phase contrast or partial-Fourier acquisition.
The present invention uses phased array combining to cancel ghosts by a variety of distortion mechanisms, including space-variant distortions, such as local flow or off-resonance. The method uses a constrained optimization that optimizes signal-to-noise ratio (SNR) subject to the constraint of nulling ghost artifacts at known locations. The method may be applied to cancel ghost artifacts that result from a variety of phase encode strategies, for example, multi-shot EPI with non-interleaved phase encode acquisition. The overall strategy of using phase encode acquisition orders with distortion that results in ghosts, followed by applying this phased array ghost cancellation method has a number of benefits, including reduced blur and geometric distortion, reduced acquisition time (eliminating echo shifting), and reduced sensitivity to flow. This method may be used in conjunction with phase sensitive techniques.
In one aspect multi-coil, full field-of-view k-space data is passed through a converter (or image reconstructer) to convert the k-space data to image domain. After the conversion, the images contain ghost artifacts. The images are then passed through one or more phased array combiners. The phased array combiners act to separate the superimposed images (desired and ghosts). Each phased array combiner in conjunction with shifting the input image produces an image or ghost with the other ghost images cancelled. Alternatively, the phased array combiner coefficients may be shifted rather than the input images to similarly separate the superimposed images. The outputs of the phased array combiners each represent images without ghost distortion. Each output image may have a different amplitude or complex weighting, where the weighting function may vary pixel to pixel in the image. One or more individual images (ghosts) may be combined to produce a final image with ghost artifacts cancelled. In one method, the individual images may be combined non-coherently to produce a magnitude image. In an alternative embodiment, the outputs may be coherently combined by means of complex weightings to produce a complex image, which may either be magnitude detected or used in conjunction with other techniques which require a complex image.
In another aspect, the phased array combiners coefficients used to cancel ghost artifacts can be calculated adaptively or dynamically. There are a number of ways to perform the adaptive calculation of the phased array combiner coefficients. One technique is to reconstruct an artifact-free lower temporal resolution image from a time sequence of multi-coil, k-space data in which the phase encode order is time varying in a specific fashion. The reconstructed artifact free image is then used to calculate the phased array coefficients that are applied to the phased array combiners.
In yet another aspect, the output combiner coefficients may be calculated adaptively to produce the final sequence of images with ghost artifacts cancelled.
The invention provides a number of benefits including reduced distortion due to off-resonance, in-plane flow, and EPI delay misalignment, and the elimination of the need for echo-shifting.
Further features and advantages of the invention will become apparent with reference to the following detailed description and accompanying drawings.